This invention relates to electro-optic devices and, more particularly, to liquid crystal displays.
Many liquid crystal displays have been devised since the properties of liquid crystal materials were found to be useful in display technologies. One of the more popular types of liquid crystal displays utilizes the properties of nematic-phase liquid crystal materials to form a twisted nematic display operating in the reflective mode. A cell is constructed in which the nematic material is sandwiched between front and rear transparent substrates which have received one or more forms of surface treatment and which are oriented in a particular relationship to one another so as to produce an optical media which in its relaxed state rotates the plane of polarization of light passing through it by ninety degrees. Such an arrangement generally utilizes a first absorbing type polarizer which restricts the light entering and leaving the front of the cell to that having a particular plane of polarization. A second absorbing polarizer having its plane of transmission rotated ninety degrees with respect to the first polarizer is positioned behind the liquid crystal cell, and a mirror is positioned behind the second polarizer.
The light passing through the first polarizer and the front substrate has its polarization plane twisted by the liquid crystal material so that the light exiting the cell passes through the second polarizer, is reflected by the mirror, and travels back through the same path to appear as a bright area on the display.
The two substrates are each provided with transparent electrodes. A voltage placed across these electrodes provides an electric field which causes the liquid crystal molecules to rotate, and thus untwist. Under this condition, light passing through the first polarizer and into the liquid crystal material does not pass through the second polarizer, since its polarization plane is no longer twisted into alignment with this polarizer. Instead, the light is absorbed by the second polarizer, creating a dark area on the display. By appropriate shaping and actuating of the electrodes, various alphanumeric and other images may be shown on the display. Display devices of the type described above are used in watches and, more recently, in flat panel displays used with computers, especially portable computers.
Conventional liquid crystal devices exhibit relatively low contrast between activated and nonactivated portions when the ambient light level is low; reading such a display becomes difficult. An additional problem associated with liquid crystal displays is due to the fact that the polarizers are placed outside the liquid crystal cell, and, hence, are separated from the liquid crystal material by the thickness of the substrates. This separation contributes to substantial parallax which tends to restrict the viewing angle and reduce the resolution of the display.
Accordingly, it is a purpose of the present invention to provide a new and improve liquid crystal device.
It is another purpose of the present invention to reduce the complexity of liquid crystal displays.
It is yet another purpose of the present invention to improve the contrast and resolution of nematic-phase liquid crystal displays.